i have know how on programing
Then you would know that the program would be little more than
if (voltage < 24) {
pulse_relay_set_pin();
beep_alarm();
}
if (voltage > 30)
pulse_relay_reset_pin();
Normally some hysterisis would be a good idea as well but in this case I don't think it matters as doing multiple pulses to the same latching relay pins won't do any harm.
As mentioned divide your input voltage by 6 so the Arduino sees 0-5v.
And for the PC interface read the RX line and test for one or more characters.
BTW, this could be done with a comparator or two or a single 8-pin ATtiny85, an Arduino is an overkill.